The invention relates to a sheet feeder comprising a suction head, which is provided with separating suckers arranged on a vertically movable support, and at least one row of drag suckers mounted on a support, which is adapted to be reciprocated partly at the same speed as conveyor means for further transport and is preferably able to be pivoted for righting oblique sheets, such drag suckers being adapted to be supplied with vacuum on the transfer of a sheet and to be vented on the release of the sheet from the separating suckers to such means for further conveyance thereof.
In known arrangements of this type the venting of the drag suckers is performed via a central suction air control valve arranged in the suction head and which is controlled by means of a separate drive cam. However adjacent to this suction air control valve there are normally only comparatively small cross sections for venting. Furthermore owing to the large distance between the suction air control valve and the drag suckers there is a large dead space contained in the ducts, which has to be vented. Therefore in the case of the conventional arrangements there is a only a sluggish venting action and drop in the level of the vacuum, more particularly since the degree of vacuum is comparatively low. These features mean that in the case of transfer at the same speed it is necessary to allow a long period in which the drag suckers and the timing roll accepting the sheet from them move at the same speed in accordance with the time of transfer of the products at all speeds and this means that the drag suckers have to be smartly accelerated and slowed down. This in turn means that, more particularly when the machine is running fast, there will be sudden loading and jerky movement of the mechanism leading to rough and irregular performance likely to involve failure.
Although it is conceivable that the venting of the drag suckers might be timed to start ahead of sheet transfer, this would involve a modification of the timing cam for the suction air valve. Furthermore such a change in the cam would make operation dependent on the speed of the machine owing to the use of a single turn shaft to drive the cam. It would seem that this is the reason that so far this modified design has not been accepted and the disadvantages of the prior art are tolerated.